1. Field of the Invention
This invention relates to a ceramic welding process and to a lance suitable for use in such a process.
2. Background of the Art
Previous ceramic welding processes have been described in Glaverbel's British Patents Nos 1,330,894 and 2,170,191.
Ceramic welding is particularly suited to the in situ formation of a refractory mass on a refractory wall of furnaces or other refractory apparatus for the hot repair of the wall. It is preferably implemented when the wall is substantially at its normal operating temperature. It is particularly useful for repairing or reinforcing the walls or wall linings of glass melting furnaces, coke ovens, cement kilns or furnaces used in the petrochemical industry, or refractory apparatus used in ferrous or non-ferrous metal metallurgy. Moreover, the repair can sometimes be carried out during the operation of the furnace, for instance for the repair of a glass melting furnace superstructure, or during the normal operating cycle of the refractory article, for example a steel-pouring ladle can sometimes be repaired within the normal interval between teeming and recharging. The process is also useful for the formation of refractory components, for instance for the surfacing of other refractory substrates.
In the ceramic welding process as practised, a mixture of refractory particles and fuel particles (the "ceramic welding powder") is conveyed from a powder store along a feed line to a lance from which it is projected against a target surface. The carrier gas which leaves the lance outlet with the ceramic welding powder ("the carrier gas") may be pure (commercial grade) oxygen, or it may comprise a proportion of a substantially inert gas such as nitrogen, or indeed some other gas. In any event, the carrier gas leaving the lance outlet with the ceramic welding powder contains at least sufficient oxygen for substantially complete combustion of the fuel particles. It is by no means essential that the gas stream into which the welding powder is introduced from the feed store should have the same composition as the carrier gas which leaves the lance outlet. Some, or indeed all, of the required oxygen in the carrier gas may be introduced into the feed line at one or more locations between the powder introduction point and the outlet of the lance. The fuel used essentially consists of particles of a material which is capable of being oxidized exothermically to form a refractory oxide product. Examples of suitable fuels are silicon, aluminium, magnesium, zirconium, and chromium. Such metallic fuels may be used alone or in combination. The fuel burns and heat is liberated by its combustion to melt at least the surfaces of the refractory particles so that a strongly coherent refractory weld mass is formed which adheres well to the target surface.
It is common practice to select the ceramic welding powder in such a way that the weld deposit formed has a chemical composition which is approximately the same as that of the target surface. This assists in reducing thermal shock at the interface between a repair weld and the repaired refractory due to temperature cycling of the furnace. Such selection of the welding powder also helps to ensure that the refractory quality of the weld mass is sufficiently high for the location where that repair is made. Of course it is also known to select the ceramic welding powder in order to form a repair or lining of higher grade than the refractory on which the weld is formed.
When forming a refractory mass by ceramic welding, a certain amount of porosity may be incorporated in the weld mass. The extent of such porosity is dependent in part on the skill of the welder, and on the conditions under which the welding operation is carried out. Such porosity may be tolerable, indeed in some circumstances it may be advantageous, since a high degree of porosity promotes thermal insulation. However, an excessive degree of porosity may be objectionable at furnace locations where the refractory is subjected to particularly severe corrosive action, and especially the corrosive or erosive action of molten material contained within the furnace. The degree of porosity which is acceptable in a given piece of refractory material depends on the inherent refractoriness of that material and on the conditions to which it will be subjected in use.
The present invention results from research into the formation of a refractory lining or repair on parts of apparatus which are particularly likely to undergo intense erosion. This erosion may be due in particular to mechanical or thermo-mechanical abrasion, or to liquid or gaseous phase corrosion of the material forming the wall, or may be due to a combination of these effects.
One example of such a requirement for good resistance to a tendency to intense erosion is in the field of glass melting furnaces. The inner surface of the tank blocks of a glass melting furnace at the location of the surface of the molten glass bath provides a particular example of a refractory surface subject to very intense corrosive action. The tank block surface erodes very rapidly to such an extent that half the thickness of the blocks may be readily and comparatively rapidly eaten away at this location. This erosion is known by the technical term "flux line corrosion". Tank blocks subject to very high temperatures, such as the tank blocks of the melting and refining zones of the furnace, are conventionally formed of highly refractory materials such as refractory materials containing a high proportion of zirconia. Even so, they have to be continuously and vigorously cooled to lessen the erosion.
Other examples of refractories which are subject to risk of particularly severe erosion are casting orifices or ladles used in the manufacture or transport of molten metals, for example torpedo ladles, as used for instance in the iron and steel industry, copper smelting and refining furnaces, converters such as those used in steelmaking or in the non-ferrous metals industry. Cement kilns may also be mentioned here.
It is a principal object of this invention to provide a new ceramic welding process which facilitates the formation of high quality refractory weld masses which exhibit good resistance to erosion and corrosion.